regarding the chinese 6040 routers:
Cheap chearful and not very sturdy. Backlash is whatever you get from a C7 no preload screw which 0-0.05mm ish if you are lucky 0.1+ if not.

With a little bit of effort you can make the machine perform well in my opinion. Firstly you will need to add limit switches, a decent motion controller, change the cables out for better quality ones. Then you should tackle the distinct lack of rigidity of the ground unsupported rails. If you get one with an all-in-one stepper driver board, set money aside for when it blows smoke.

Aircooled spindles do not allow you to go at low RPM as they overheat. Also they sound like a concorde taking off...apparently. Most of the 6040s come with water cooled spindles anyway. Most of them with ER16 collets too so your 8mm endmill will be fine.

Plenty of people with these machines on youtube to give you advice on how best to tackle the teething issues.

If you want a machine that is ready out of the box though, best look elsewhere.